It is common knowledge that heavy duty internal combustion engines using aluminium alloy pistons are subject to the difficulty that walls of the piston ring grooves cut in the piston head are subject to extreme and deformation in use, thus causing engine failure. Because of this, reinforcement of the ring groove area has become a necessity.
At present, much attention is centered on the provision of ring groove reinforcements by spraying a wear-resistant metal into the preformed peripheral recess whose dimensions exceed the final dimensions of the groove receiving the piston ring.
There is well known a method of making an aluminium alloy piston with a ring groove reinforcement (cf. French Pat. No. 2122,4). Among other things, this method ensures the reinforcement of the top ring groove provided in the aluminium alloy piston. Experience has shown that in the course of operation of internal combustion engines, the top ring grooves show the worst wear. The first step of the method in question consists of forming a peripheral recess in the piston body adjacent its top face, the dimensions of said recess exceeding the final dimensions of the groove for receiving the top piston ring. Then the piston body is heated up to a temperature ranging from 165.degree. to 200.degree. C. and, while the temperature of the piston body is maintained in the stated range, a bonding metal is sprayed on the surface of the peripheral recess, whereupon a wear-resistant metal is sprayed into the peripheral recess to fill it, the temperature of the piston body being within the stated range.
In this method, nickel aluminide is used as a bonding metal, and stainless steel is used as a wear-resistant metal. The wear-resistant metal layer is bonded through the bonding metal to the aluminium alloy of the piston body.
After cooling the piston body, the final step is performed consisting in machining in the wear-resistant metal a groove for receiving the top piston ring of the specified size.
Said method improves significantly the ring groove reinforcement in the piston body but is a very labour-consuming process because of the necessity to perform, prior to forming the ring groove, the peripheral recess and to spray the bonding metal over the surface of said recess.
Moreover, the bonding metal layer fails to provide a sufficient strength of the bond between the piston alloy and the sprayed wear-resistant metal, since the piston under operation is subjected to thermal and mechanical stresses, which, in turn, results in the formation of cracks in the bonding metal layer, thus causing chipping and flaking off of the wear-resistant metal layer and, finally, piston failure.
It is also known to provide the ring groove reinforcement in an aluminium alloy piston by means of an insert formed as an annular wear-resistant weld deposited in the piston body.
Known to the prior art is a method of reinforcing an aluminium alloy piston ring groove (cf. U.S. Pat. No. 3,014,771) and in particular, of reinforcing a top ring groove in such a piston.
To provide the top ring groove reinforcement by this method, an annular recess is cut adjacent the piston top face, the dimensions of the annular recess exceeding the final dimensions of the top ring groove to be formed. Thereupon, an annular wear-resistant weld is formed by depositing a wear-resistant metal within the annular recess, thus forming an insert having particles of the wear-resistant metal.
The weld may be deposited by gas welding or by electric arc welding with a filler rod having a matrix of an alloy that should bond securely to the material of the piston body.
In the case of an aluminium alloy piston, the rod matrix should be a related aluminium alloy containing particles of hard durable material, such as ferrous alloy, evenly dispersed therein.
Thus, the weld matrix is a related aluminium alloy which is bonded securely to the piston alloy.
Moreover, the desired durability is imparted to the weld by the ferrous particles evenly dispersed in the aluminium alloy of the weld matrix.
The machining of the top ring groove into the weld can be done by conventional means.
As compared to the above method, the method in question enables the strength of the bond between the piston alloy and the weld material to be increased, since the weld is formed with a related aluminium alloy.
However, this method is also a labour-consuming process because of the necessity to preform the annular recess in the piston body.
Furthermore, during the deposition of the weld in the annular recess of the aluminium alloy piston body, inevitable defects peculiar to the welding of aluminium alloys occur in the fusion area, which defects reside in gas and oxide inclusions and in faulty fusion, and are the cause of the generation of stress concentrations initiating cracks in the fusion area under the action of thermal and mechanical stresses.
This decreases drastically the strength of the bond between the weld material and the piston alloy, thus causing the piston failure.